Hitherto, surface protective films have been extensively used in the applications for preventing formation of scratches or deposition of contaminants on resin plates, metal plates, glass plates, etc., upon transportation, storage or processing thereof, preventing formation of scratches or deposition of dirt and dusts or contaminants on members used in electronics-related fields such as liquid crystal display panels and polarizing plates upon processing thereof, preventing deposition of contaminants on automobiles upon transportation or storage thereof or protecting automobile painting against acid rain, protecting flexible printed boards upon plating or etching treatments thereof, and the like.
It has been required that these surface protective films can exhibit an adequate adhesion strength to various kinds of adherends such as resin plates, metal plates and glass plates upon transportation, storage or processing thereof, can be attached onto these adherends to protect the surface thereof, and can be easily peeled off from the adherends after accomplishing the objects as aimed. In order to overcome these problems or tasks, the use of polyolefin-based films for the purpose of protecting the surface of the adherends has been proposed (Patent Literatures 1 and 2).
However, since the polyolefin-based films are used as a base material of the surface protective films, it is not possible to avoid occurrence of defects generally called fisheyes, i.e., formation of gels or deteriorated products derived from raw materials of the base material of the film. For example, there tends to arise such a problem that when testing the adherend onto which the surface protective film is kept attached, these defects on the surface protective film are detected as defects of the adherend, etc., thereby causing disturbance of the test.
In addition, the base material for the surface protective films is required to have a certain degree of mechanical strength to such an extent that the base material is free of expansion owing to a tensile force applied upon various processing steps such as lamination onto the adherend, etc. However, the polyolefin-based films are generally deteriorated in mechanical strength, so that there tends to occur such a problem that the films are unsuitable for undergoing high-tension processing steps in association with increase in film-processing velocity, etc., which must be conducted in view of the importance to productivity of the film.
Further, in the case where the processing temperature of the polyolefin-based films is increased for enhancing processing velocity or improving various properties thereof, the polyolefin-based films tend to suffer from deterioration in dimensional stability owing to poor shrink stability upon heating the films. For this reason, there is an increasing demand for films having not only less heat deformation but also excellent dimensional stability even when subjected to high-temperature processing.
In general, plastic films have drawbacks such as deposition of dirt and dusts thereon owing to generation of static electricity upon peeling or by friction (delamination electrification or frictional electrification). In particular, when the plastic films are used in the applications in which inclusion of foreign matters should be avoided, for example, as a protective film in the processes for production of optical members such as polarizing plates, significant problems tend to occur.